The cleansing of skin with surface-active cleansing preparations has become a focus of great interest. Many people wash and scrub their skin with various surface-active preparations several times a day. Ideal skin cleansers should cleanse the skin gently, causing little or no irritation, without defatting and overdrying the skin or leaving it taught after frequent routine use. Most lathering soaps, liquids and bars included, fail in this respect.
Synthetic detergent bars, frequently referred to as "syndet bars", are well known and are becoming increasingly popular. However, widespread replacement of soap bars by syndet bars has not so far been possible for a variety of reasons, primarily the poor physical characteristics of syndet bars as compared to soap bars. In clear distinction from syndet bars which are "soap-free" are the so-called "combo" bars which are combinations of fatty acid salts and synthetic detergents. One type of combo bar is produced by combining fatty acid soaps with salts of acyl isethionates. These combo bars, because they contain both a synthetic detergent and a soap component, do not suffer from the poor physical characteristics of syndet bars, and are milder to human skin than traditional fatty acid soaps.
However, a problem encountered in manufacturing combo bars relates to the incorporation of the synthetic acyl isethionates surfactant into the fatty acid soap. Acyl isethionates are high melting solids with low solubility in either water or organic solvents. Aqueous solutions of sodium cocoyl isethionate are inherently highly viscous. Thus, it is very difficult to incorporate acyl isethionates into soap formulations without the attendant physical disadvantages associated with their poor handling properties.